with regard to internal temps, I will say that Defenders tend to not come with any factory cab insulation, probably due to their utilitarian nature. Nor do they come with any real window tinting. The Disco's get both. When I first got my Defender, I experienced all the same issues mentioned above: not retaining heat in winter, or staying cool in summer. I did two things to change all that. I put luxury sound and thermal insulation throughout my cab. The roof lining alone made a HUGE difference in the winter. I also just recently did window tinting; 35% in front doors, 5% in middle doors, and 2.5% (limo tint) in all rear windows). The insulation job I did made it very easy to heat up and maintain heat throughout the cab. Last winter, we went up to snow country in the mountains. Above the snow line, my wife had me turn the heater off even though it was around freezing outside, because the heater was making it too warm even on lowest setting. We could drive for hours without the heat on because just our body heat was enought to maintain the temp in our now insulated cabin. The A/C's biggest boost however was the tinting. All those windows in a 110 greenhouse pretty well, but as it started to warm up here in the Carolinas, I decided it was time to do the windows before another gruelling summer. it's mid 80's now, and the tinting has proved so effective, that rolling around town in mid 80's, I don't want my A/C on past the lowest setting because it gets too cool.
yes, the investment has paid off in those mods. Aluminum body shells aren't less thermally effecient than steell ones. The difference is in what you do with it. Land Rover offers a blank palette when it comes to the Defender. Paint what you will.